Evolution of Telomeres in Schizosaccharomyces pombe and Its Possible Relationship to the Diversification of Telomere Binding Proteins

Abstract

Telomeres of nuclear chromosomes are usually composed of an array of tandemly repeated sequences that are recognized by specific Myb domain containing DNA-binding proteins (telomere-binding proteins, TBPs). Whereas in many eukaryotes the length and sequence of the telomeric repeat is relatively conserved, telomeric sequences in various yeasts are highly variable. Schizosaccharomyces pombe provides an excellent model for investigation of co-evolution of telomeres and TBPs. First, telomeric repeats of S. pombe differ from the canonical mammalian type sequence. Second, S. pombe telomeres exhibit a high degree of intratelomeric heterogeneity. Third, S. pombe contains all types of known TBPs (Rap1p [a version unable to bind DNA], Tay1p/Teb1p, and Taz1p) that are employed by various yeast species to protect their telomeres. With the aim of reconstructing evolutionary paths leading to a separation of roles between Teb1p and Taz1p, we performed a comparative analysis of the DNA-binding properties of both proteins using combined qualitative and quantitative biochemical approaches. Visualization of DNA-protein complexes by electron microscopy revealed qualitative differences of binding of Teb1p and Taz1p to mammalian type and fission yeast telomeres. Fluorescence anisotropy analysis quantified the binding affinity of Teb1p and Taz1p to three different DNA substrates. Additionally, we carried out electrophoretic mobility shift assays using mammalian type telomeres and native substrates (telomeric repeats, histone-box sequences) as well as their mutated versions. We observed relative DNA sequence binding flexibility of Taz1p and higher binding stringency of Teb1p when both proteins were compared directly to each other. These properties may have driven replacement of Teb1p by Taz1p as the TBP in fission yeast.

Composition and Function of Telomerase—A Polymerase Associated with the Origin of Eukaryotes

“…The lengths of telomere arrays can also vary at the level of the species or ecotypes (reviewed in [22,23]). The human-type (TTAGGG) n telomeric sequence is conserved across several eukaryotic 'supergroups' [24], including the Amorphea supergroup with metazoan and fungal species (reviewed in [25,26]). At the same time, several exceptions are known across the Amorphea supergroup, e.g., in insects (TTAGG) n [27,28], in Nematodes (TTAGGC) n [29] or in several fungal genera, where very complex or irregular telomeric motifs were described [26,30] (Figure 2).…”

“…The human-type (TTAGGG) n telomeric sequence is conserved across several eukaryotic 'supergroups' [24], including the Amorphea supergroup with metazoan and fungal species (reviewed in [25,26]). At the same time, several exceptions are known across the Amorphea supergroup, e.g., in insects (TTAGG) n [27,28], in Nematodes (TTAGGC) n [29] or in several fungal genera, where very complex or irregular telomeric motifs were described [26,30] (Figure 2). Additionally, the telomeres of some insects are constituted with unusual telomeric motifs, or even with telomeric repeats that consist of arrays of non-long-terminal-repeat (non-LTR) retrotransposons [31][32][33].…”

A Matter of Scale and Dimensions: Chromatin of Chromosome Landmarks in the Fungi

“…Candida albicans has longer, 23-nt tandem repeats that are overall GC-rich and contain a 5′-TGGTGT-3′; this arrangement is also found in other Candida species [65, 66] (Figure 2). The canonical tandem repeat in S. pombe is G 0-4 GGTTACAC 0-1 [67, 68], while the animal and human pathogens in the Taphrinomcyotina, Pneumocystis carinii and P. jirovecii , have telomere repeats composed of the more common 5′-TTAGGG-3′ sequence [69, 70]. Telomeres from Dikarya in classes outside of the Taphrinomycotina and Saccharomycotina were found to be 5′-TTAGGG-3′ repeats, like in N. crassa [71, 72] and A. nidulans [73, 74]; many additional species have been examined as part of genome sequencing projects (Figure 1), though many projects did not capture any telomere repeats, e.g.…”

Commentary: Single-stranded telomere-binding protein employs a dual rheostat for binding affinity and specificity that drives function

“…The situation is even more pronounced in species possessing heterogeneous telomeric repeats such as Saccharomyces cerevisiae and Schizosaccharomyces pombe . This poses a great challenge to TBPs, whose binding properties must rapidly co-evolve with their DNA targets (Steinberg-Neifach and Lue, 2015; Sepšiová et al, 2016; Červenák et al, 2017).…”

“…Over the past few years this question has been addressed through biochemical and genetic analysis of TBPs from various yeast species (Kramara et al, 2010; Lue, 2010; Visacka et al, 2012; Steinberg-Neifach and Lue, 2015; Sepšiová et al, 2016; Červenák et al, 2017). Our studies have shown that Tay1, the dsTBP of the yeast Yarrowia lipolytica (Kramara et al, 2010) exhibits lower affinity for its own telomeres than for the mammalian-type telomeric repeats (Table 1).…”